Discovered from the hypothalamus, thyrotropin-releasing hormone (TRH, L-pyroglutamyl-L-histidyl-L-prolinamide) is a peptide that promotes secretion of thyroid stimulating hormone and prolactin of the pituitary glands (Schally, A. V. et al., “Isolation of thyrotropin releasing factor (TRF) from porcine hypothalamus,” Biochem. Biophys. Res. Commun., 25(2):165–169 (1966)). Specifically, TRH is a tripeptide with blocked N- and C-terminal residues that is considered common to mammalian species including man, having a structure as shown below:
There are three functional portions in the TRH tripeptide structure. The right portion of the molecule is known to those skilled in the art as the “prolineamide” or “C-terminal prolineamide” portion, the center portion of the molecule is known as the “histidyl” or [His2] portion, and the left portion of the molecule is known as the “pyroglutamyl” amino- or “N-terminal” portion.
A major percentage of TRH is released from the hypothalamic nerve terminals in the median eminence to stimulate the secretion of thyroid stimulating hormone. TRH is reported as being widely distributed in almost all sites of the brain and throughout the central nervous system as well as in a variety of tissues, including the alimentary tract, pancreas, placenta, retina of the eye, intestines, and adrenals (Morley, J. E. et al., “Extrahypothalamic thyrotropin releasing hormone (TRH)—its distribution and its functions,” Life Sciences, 25(18):1539–1550 (1979)).
The distribution of TRH throughout the brain and various organs suggests that TRH plays an important role in the function of the central nervous system and in endocrine-related biological activity. In fact, endogenous TRH is reported as having the ability to act as either a neurotransmitter or a neuromodulator or both.
TRH has recently been shown to antagonize many of the effects of the endogenous opiates in spinal cord injury (Faden, A. I. et al., “Thyrotropin-releasing hormone improves neurologic recovery after spinal traumas in cats,” N. Engl. J Med., 305(18):1063–1067 (1981)). The advantage of TRH is that it acts as a physiological opiate antagonist without affecting nociception.
Central or peripheral administration of TRH to organisms has been found to bring about various central nervous actions and behavioral effects in animals (Guillemin, R., Recent Prog. Horm. Res., 33:1–28 (1977)). For example, a central nervous system effect of THR is the “analeptic action,” which is the reduction of barbiturate narcosis or haloperidol-induced catalepsy as a measure of cholinergic stimulation (Schmidt, D. E., “Effects of thyrotropine releasing hormone (TRH) on pentobarbital-induced decrease in cholinergic neuronal activity,” Commun. Psychopharm., 1(5): 469–73 (1977); Sharp, T. et al., “Analeptic effects of centrally injected TRH and analogues of TRH in the pentobarbitone-anaesthetized rat,” Neuropharmacology, 23(3):339–48 (1984); Horita, A., “An update on the CNS actions of TRH and its analogs,” Life Sci., 62(17–18):1443–8 (1998)).
Additionally, the administration of TRH has been reported to have therapeutic effect on various disorders and conditions, such as schizophrenia (Inagata, K. et al., “Behavioral effects of protirelin in schizophrenia,” Arch. Gen. Psychiatry, 35(8):1011–1014 (1978)), melancholia (Hatanaka, C. et al., “An improved synthesis of thyrotropin releasing hormone (TRH) and crystallization of the tartrate,” Biochem. Biophys. Res. Commun., 60(4):1345–50 (1974)), spinocerebellar degeneration (Sobue, I. et al., “Effect of thyrotropin-releasing hormone on ataxia of spinocerebellar degeneration,” Lancet, 1(8165):418–9 (1980)), and neurologic disorders. Improvement of disturbance of consciousness has also been associated with the administration of TRH.
Unfortunately, the clinical utility of TRH has been limited by its rapid metabolism and clearance and poor access to the brain. Due to its highly water-soluble nature and in combination with the absence of specific transport systems in endothelial cells that form the blood-brain barrier (BBB), TRH has poor access to the central nervous system. Moreover, its endocrine effect (i.e., elevation of thyroid hormone levels) is usually manifested at doses in which significant cognitive improvement is observed.
In addition, TRH exhibits a very short biological half-life, roughly 4–6 minutes in rats and humans (Bassiri, R. M. and R. D. Utiger, “Metabolism and excretion of exogenous thyrotropin-releasing hormone in humans,” J. Clin. Invest., 52(7):1616–1619 (1973); Morley, J. E. et al., “Plasma clearance and plasma half-disappearance time of exogenous thyrotropin-releasing hormone and pyroglutamyl-N3im-methyl-histidyl prolineamide,” J. Clin. Endocrin. Metab., 48, 377–380 (1979); Duntas, L. et al., “Pharmacokinetics and pharmacodynamics of protirelin (TRH) in man,” Dtsch. Med. Wschr., 113(35):1354–1357 (1988); Iversen, E., “Intra- and extravascular turnover of thyrotropin-releasing hormone in normal man,” J. Endocrin., 118(3):511–516 (1988)) due to rapid degradation by endogenous enzymes, in particular by pyroglutamyl aminopeptidases (Bauer, K. et al., “Specificity of a serum peptidase hydrolyzing thyroliberin at pyroglutamyl-histidine bone,” Euro. J Biochem., 118(1):173–176 (1981); and Bauer, K., “Degradation and biological inactivation of thyrotropin releasing hormone (TRH): regulation of the membrane-bound TRH-degrading enzyme from rat anterior pituitary by estrogens and thyroid hormones,” Biochimie, 70(1):69–74 (1988)).
The short half-life of TRH is most likely due to rapid degradation of the peptide at both the carboxy (COOH—) and amino (NH2) termini of the TRH molecule. Cleavage of the pyroglutamyl moiety of TRH by peptidases causes formation of the metabolite cyclo-histidyl-proline-diketopiperazine. Deamidation of TRH results in the formation of the free acid TRH-OH.
There have been numerous attempts to produce TRH-like compounds to separate the central nervous system (CNS) and hormonal effects. Analogues where [His2] is replaced by an aliphatic amino acid residue, such as by leucine (Leu) or narvaline (Nva), are characteristic representatives. While these compounds have improved metabolic stability and somewhat higher lipid-solubility compared to TRH, their peptide character prevents them from satisfactory access to the CNS.
Invasive (i.e., by-passing or altering the blood-brain barrier (BBB)) and non-invasive strategies have been developed for improving CNS drug-targeting of hydrophilic drugs such as neuropeptides. A non-invasive strategy is to provide a lipophilic prodrug of a parent drug. Due to improved lipid-solubility, the prodrug may penetrate biological membranes including the BBB and convert the pharmacologically active species at the site of action via predictable enzymatic and/or chemical transformation. Although a lipid-soluble prodrug may assure the diffusion of the parent drug through the BBB, efflux from the CNS is still not prevented if the prodrug cannot be adequately converted into the parent drug at the target site.
Therefore, there is a need for therapeutic agents that are pharmaceutically effective at those regions where they are required. More importantly, there is a need for therapeutic agents that are not rapidly metabolized, effectively penetrate the BBB, are therapeutically active at the blood-CNS interfaces, and do not manifest an unwanted endocrine effect.